Every navigator is familiar with the sight of two intersecting course lines on a nautical chart. And every navigator knows that no ship can sail through the intersection. To get from one course line to the other the ship has to follow some other line for some time. For small ships this is not important, but for large ships the transitory line and the time sailed on it have gradually become more interesting for several reasons.

To what extent, if at all, were globes used for practical navigation during the sixteenth and early seventeenth centuries? Globes were discussed by several writers of the period, but, with the exception of John Davis, these were not practical seamen. What evidence is there that globes were ever actually used by navigators at sea?

The first recorded globe was made by Martin Behaim in 1490. The most celebrated makers who followed were Gemma Frisius in 1530, Gerhard Mercator in 1541 and Emery Molyneux in 1592.

Navigation lights for ships were first regulated in the United Kingdom by a statute introduced in 1848 and revised four years later, since when there have been few changes. Originally the regulations referred to oil-burning lamps fitted to vessels rarely in excess of 400 tons and rarely sailing much above 10 knots. The sheer increase in average size and in speed of typical modern ships, particularly marked over the last four decades, raises the question as to how effective the regulation lights actually are in practice. This question is made more pertinent in view of the other great change in ships, namely their general arrangement: the superstructure may be forward, midships or astern, masts and derricks vary in number, complexity and size. Although the regulations for the positioning of lights are reasonably comprehensive they inevitably leave considerable freedom to the designer because of the variety of superstructure configurations.

D. H. Sadler has pointed out that the method I proposed in my paper (34, 247) can be improved, in his view, by the use of the formula

(1 + f) × distance = D.lat × sec(Co)

where the tabulation of Jean be found in his earlier paper (9, 371).

In the eighth Duke of Edinburgh Lecture, presented to the Institute in 1976, Professor Thorn discussed the extent of Megalithic man's knowledge of astronomy and from surveys of ancient sites showed how a calendar was produced and sensitive methods of astronomical observation developed. Mr Watts suggests that the positions of ancient sites in the area he has examined show an early familiarity with the concept of parallels and meridians.

It is not normally assumed that latitude and longitude were in use in times as remote as the late Neolithic and early Bronze Ages. The New Forest with its relatively sparse set of major earthworks is, however, a good area to illustrate that ancient sites are set with careful regard for both latitude and longitude, arid particularly the latter.

The first point to note about the North Atlantic air traffic system is the diurnal pattern of flow. This is strongly tidal, with westbound flow predominant during daylight hours and eastbound flow overnight. There is some traffic which flows against the main stream, particularly during the day, but the strong tendency for one-way traffic results from the difference in local times on opposite sides of the ocean, together with the transit times of flight at current speeds. This means that the general pattern will probably persist, unless and until aircraft speeds change radically.

An experiment is described in which colour-coded information concerning the aspect of target ships was provided in addition to a relative-motion radar display. This was compared to the normal relative-motion display alone, while a third condition used the normal true-motion display alone. The performance of twenty-one experienced seafarers on six exercises was assessed by five expert judges as an index of display effectiveness. The method of evaluation was found useful and practical, while user reports were shown to be inappropriate. No overall difference between the three displays was found; the reasons for this are discussed and the conclusion is drawn that training is required to gain the benefit of colour in a display.

This paper outlines the phase of the flight planning operation in which airlines calculate minimum-time tracks for the purpose of advising oceanic control centres of their needs prior to the definition of the Organised Track System (OTS). The results are based on 12- and 24-hour forecasts for a level of 250 mb and aircraft assumed to fly at a constant Mach number of 0·82. The problem is defined in terms of a network in which the domestic route structure on each side of the North Atlantic is represented.

In a previous paper a model was outlined for an encounter between two vessels. This paper shows how the model has been developed to include the simulation of the behaviour of more than two ships, the entrance to harbours, and narrow channels.

In a previous paper by the authors the concept of a domain was examined. Goodwin's definition of a domain was used, namely, ‘the effective area around a ship which a navigator would like to keep clear with respect to other ships and stationary objects’. The concept of distinct sectors for sidelights and sternlight was modified mathematically so that an area equal to the total of the segments was contained within a circle. By off-centering the position of the ship within this circle, the weighting of the differing areas for the various sectors was retained (Fig. I). A second circle with the ship off-centre was introduced called an arena or ‘sphere of influence’. When a ship is inside the arena a navigator becomes aware of the other ship and decides what action, if any, is needed to keep his own domain unviolated. This resulted in a model which obeyed the Collision Regulations.

Having been concerned in the mid-fifties with developing the general equations of most economic flight and applying them to the profitability of an airline, I was surprised to discover when I was a ship financier in the early seventies that the much simpler equivalent marine problem had never been addressed by our clients, nor did the question arise apparently in the negotiation of their time charters with oil companies which are not conspicuously unsophisticated in some matters. I was therefore most interested to read Captain P. M. Alderton's paper on ‘The optimum speed of ships’, this Journal, 34, p. 341. If it were not for his charming reference to Napier's paper on the same theme dated 1865 one would be tempted to think that this was a case where the air leads the sea by decades.

We are still in the process of collecting and developing ways of studying and analysing air traffic routes across the North Atlantic. The results presented in this paper must therefore be recognized as provisional. The data comprise some twelve examples of North Atlantic weather forecasts issued from Bracknell; they are sent to us regularly for the 2nd and 15th day of each month. We have also made arrangements to receive notification from the Heathrow Meteorological Office of any days in which there were significant changes in the weather forecast, so that we can request the additional information from Bracknell. Each set of weather data contains the ‘analysis weather’, that is the best estimate of the actual weather at 1200 GMT, and therefore applicable to the time when aircraft are making westerly departures across the North Atlantic from European cities, and also the weather forecasts issued for 12 and 24 hours before this time.

The proposal by A. J. Wright (this Journal, vol. 34, no. 1, p. 142) that banded contour charts should be introduced, appears attractive at first sight. It appears to provide immediately recognizable indication of safe areas for the navigation of those ships whose draught is within certain limits. However, I feel that the system as proposed suffers from some major weaknesses

Allowance for height of tide. Tidal ranges of several metres are by no means uncommon. Failure to make proper allowance for this could result in a wrong assessment by the user of where navigationally safe water lies. Such tidal ranges frequently occur in areas of high-density traffic. Here, the pressures on the mariner are such that the temptation to ‘eye-ball the navigation’ is greatest – yet it is in such areas that positional accuracy based on sound chartwork procedures is vital.

‘Conflicts Between Random Flights in a Given Area’

By S. Ratcliffe and R. L. Ford

The following misprints should be corrected in the paper published in Vol. 35, No. 1, pp. 47–74 (The authors' corrections to the original proofs were inadvertently ignored in the published version):

I am reminded that this was the title of a paper by Professor W. M. Smart, published elsewhere in 1946, dealing with the rhumb line on the spheroid, by references in recent issues of this Journal to a paper of mine entitled ‘Loxodro-mic distances on the Terrestrial Spheroid’ which appeared in this Journal 32 years ago: the subject crops up every few years.